Carbon dioxide normally supplied to food and beverage customers must meet a set of purity specifications known as Enhanced Ingredient Grade (EIG). Carbon dioxide of this purity is sufficient for use in food and beverages, and most plants can produce it.
Some applications require carbon dioxide of higher purity than EIG. For example, supercritical fluid extraction and supercritical fluid chromatography require small amounts of higher purity carbon dioxide. Examples of the types of higher purity carbon dioxide include Supercritical Fluid Extraction (SFE) and Supercritical Fluid Chromatography (SFC) grades. These grades of carbon dioxide are generally supplied in cylinders.
Additional applications requiring high purity and ultra-high purity (UHP) carbon dioxide have recently been developed. These include pharmaceutical processing, semiconductor processing (photoresist removal and wafer cleaning), micro-electro-mechanical systems (MEMS) drying, and metal target cleaning.
In the past, several techniques have been used to measure impurities in carbon dioxide. One common technique is High Pressure Liquid Chromatography (HPLC). An example of this technique is described in Zito, R., “CO2 Snow Cleaning of Optics: Curing the Contamination Problem,” Proceedings of SPIE Vol. 4096 (2000). The method is time consuming, expensive, and may not be sensitive enough to detect the low levels of impurities present in UHP carbon dioxide. Further, since the nature of the contaminants in the carbon dioxide is often not known, it is difficult to design a HPLC technique that will detect all potential impurities contained in carbon dioxide.
Gravimetric methods are also currently used to determine the level of contamination contained in carbon dioxide. In one technique, a known sample volume of carbon dioxide is passed through or over a medium such as a preweighed filter used to capture solid contaminants. Alternatively, contaminants are collected in a small volume of solvent which is subsequently evaporated. The weight of contaminants is estimated by measuring the change in the mass of the filter or solvent's container. The weight of the carbon dioxide can be calculated by its flow rate or measuring the decrease in the weight of its storage cylinder. From these two measurements, the concentration of the contaminant can be calculated. Gravimetric techniques generally are labor intensive and done in a batch-mode fashion. This makes them unsuited for use in a continuous process, such as an on-line contaminant analysis.
U.S. Pat. No. 6,276,169 B1 discloses liquid carbon dioxide expanding through a primary nozzle to form a mixture of carbon dioxide snow and vapor. The contaminants contained in the liquid carbon dioxide are assumed to be trapped in the snow particles. This two-phase stream is then expanded through a secondary nozzle to produce a low density, low velocity stream. The solid carbon dioxide, with impurities, is collected at the exit of the nozzle while the carbon dioxide gas escapes. The collected frozen carbon dioxide is then heated to its sublimation point, driving off the carbon dioxide and leaving the impurities concentrated in the collection container. These impurities are then analyzed to determine their quantity and composition. For example, the carbon dioxide snow is deposited onto a high purity surface. As the carbon dioxide sublimes, the impurities are left on the surface. An ellipsometer also is used to gauge the thickness of the contamination layer on a wafer at various points, the wafer having been covered in the carbon dioxide snow collected from the nozzle. By averaging the thickness of the film over the entire wafer, a volume of contaminant is calculate which is used to estimate the impurity level. Since it collects and measures contaminants in a batch-wise fashion, this technique is not suited for continuous on-line analysis of contaminants.
U.S. Pat. No. 6,122,954 discloses measuring contaminants through the use of a surface acoustic wave (SAW) resonator. The basic idea of the SAW resonator is to measure the decrease in resonant frequency of a piezoelectric crystal onto which the contaminants deposit as a result of the sensor being at a lower temperature. Essentially, it acts as an extremely sensitive mass balance. Once the mass of the contaminant is known, its concentration can be determined. The SAW device is inefficient for measuring non-gaseous contaminants and it must be cleaned after it has sorbed a certain amount of material. SAW devices are similar to other piezoelectric techniques, but use a surface, rather than a bulk, oscillation in the crystal.
Therefore, a need exists for methods and systems suitable for analyzing impurities in carbon dioxide that reduce or minimizes the above mentioned problems.